In recent years, a high-speed communication service in conformity with a radio communication system standard called Long Term Evolution (LTE) has been started. Moreover, LTE-Advanced system, which is a developed form of LTE, is currently being discussed in 3rd Generation Partnership Project (3GPP).
For the LTE-Advanced system, a relay technique using a relay node (RN) that relays a communication between a mobile station (User Equipment, UE) and a radio base station (evolved Node B (eNodeB)) is currently under study. The radio base station eNodeB, connected to the relay node RN, is referred to as Donor eNodeB (DeNB).
With the above described relay technique currently under study for the LTE-Advanced system, the relay node RN possesses functions similar to those of the radio base station eNodeB. The functions of the radio base station eNodeB, which are possessed by the relay node RN, include a termination of wireless protocols of an Evolved Universal Terrestrial Radio Access (E-UTRA) radio interface, an S1 interface, and an X2 interface. The relay node RN can possess, for example, functions defined by the radio base station eNodeB, such as Radio Network layer (RNL), a Transport Network Layer (TNL), and the like.
Additionally, the relay node RN includes some of the functions of the mobile station UE in addition to the functions of the radio base station eNodeB. For example, as illustrated in FIG. 1, the relay node RN s1 includes functions illustrated in FIG. 1 in order to make a radio connection to a radio base station DeNB s2 and a mobility management device MME s3. Namely, the relay node RN s1 includes a physical layer (PYS) f1, a Media Access Control (MAC) f2, a Radio Link Control (RLC) f4, a Packet Data Convergence Protocol (PDCP) f3, a Radio Resource Control (RRC) f5, and a Non-Access Stratum (NAS) f6.
With the above described relay technique, E-UTRAN Radio Access Bearer (E-RAB) is set between the mobile station UE and a node (Core Node, CN) of a core network called Evolved Packet Core (EPC). Moreover, a Uu bearer is set between the mobile station UE and the relay node RN, a Un bearer is set between the relay node RN and the radio base station DeNB, and an S1 bearer is set between the radio base station DeNB and the core node CN.
In 3GPP, a consensus is reached to lay down specifications for implementing a layer 3 relay node that includes the same functions as those of a base station, such as mobility management, session setup, a handover, and the like, in LTE Release 10, which is currently being standardized.
The relay node RN can be used to expand a communication area (coverage) or increase a traffic volume. A variety of scenarios where such a relay node is installed are assumed.
As one of the assumed scenarios, a scenario where a relay node RN is mounted in a moving vehicle and connected to an optimum radio base station DeNB with a move of the moving vehicle in order to provide a communication area to a mobile station UE possessed by a passenger within the moving vehicle such as a bus, a train, or the like can be cited.
In the above described scenario where the relay node RN is mounted in the moving vehicle, the relay node RN is demanded to have a handover control procedure in order to connect to the optimum radio base station DeNB as with the move of the moving vehicle.
Conventional techniques include a technique with which a radio relay station connected to a radio base station switches a connection destination to another radio base station or a terminal connected to the radio relay station is handed over to another radio base station when a load of the radio base station exceeds a threshold value.
In addition, the conventional techniques include a technique with which a mobile base station is mounted in a vehicle set so as to interrupt a radio communication between an inside and an outside of the vehicle, and a cellular phone within the vehicle is set to communicate with a stationary base station outside the vehicle only via the mobile base station.
Furthermore, the conventional techniques include the following technique. Namely, a first radio base station notifies a relay node of specified timing when the relay node performs a handover from the first radio base station to a second radio base station. The second radio base station performs scheduling so that a downstream signal is transmitted to the relay node at the specified timing. The relay node performs scheduling so that the downstream signal is transmitted at timing other than the specified timing.
In the scenario where the relay node RN is mounted in the moving vehicle, the relay node RN is demanded to have a handover control procedure in order to connect to an optimum radio base station DeNB with the move of the moving vehicle as described above.
Namely, the relay node RN mounted in the moving vehicle and the mobile station UE within the moving vehicle travels with the move of the moving vehicle. Accordingly, the relay node RN needs to be handed over from a currently connected first radio base station Source-DeNB (S-DeNB) to a target second radio base station Target-DeNB (T-DeNB) which is newly determined to be optimum, along with the mobile station UE currently under communication.
A specific handover control procedure in the above described scenario is not standardized in 3GPP. Accordingly, the handover control procedure standardized in 3GPP, which is used in the case where the mobile station UE is handed over from the first radio base station S-DeNB to the second radio base station T-DeNB, is considered to be applied to the handover control procedure in the above described scenario.
However, if the handover control procedure standardized in 3GPP in the case where the mobile station UE is handed over between radio base stations is applied in the above described scenario, this poses the following problem associated with forwarding of user data at the time of the handover.
With the handover control procedure standardized for LTE, user data is forwarded from a first radio base station S-DeNB to a second radio base station T-DeNB when the mobile station UE is handed over from the first radio base station S-DeNB to the second radio base station T-DeNB. Namely, forwarding of the user data is performed from the first radio base station S-DeNB to the second radio base station T-DeNB so that a packet yet to be transmitted from the first radio base station S-DeNB to the mobile station UE is forwarded. With the forwarding of the user data in this way, the second radio base station T-DeNB can transmit the packet forwarded from the first radio base station S-DeNB to the mobile station UE upon completion of the handover. This can prevent the packet from being lost by the handover.
FIG. 2 is an explanatory diagram of a first case where the handover control procedure standardized in 3GPP is applied.
In a radio communication system illustrated in FIG. 2, a mobile station UE 1 is present within a communication area of a relay node RN 2, and connected to the relay node RN 2. The relay node RN 2 is present within a communication area of a first radio base station S-DeNB 3s, and connected to the first radio base station S-DeNB 3s. The first radio base station S-DeNB 3s is connected to each of a Mobility Management Entity (MME) 4 and a gateway (GW) 5. A second radio base station T-DeNB 3t is connected to each of the mobility management device MME 4 and the gateway GW 5.
In the radio communication system illustrated in FIG. 2, user data of the mobile station UE 1 under the control of the relay node RN 2 is forwarded to the relay node RN 2 via the gateway GW 5 and the first radio base station S-DeNB 3s.
Assume that the mobile station UE 1 moves from the communication area of the relay node RN 2 under the control of the first radio base station S-DeNB 3s to the communication area of the second radio base station T-DeNB 3t in the radio communication system of FIG. 2. Also assume that a handover of the mobile station UE 1 from the first radio base station S-DeNB 3s via the relay node RN 2 to the second radio base station T-DeNB 3t is caused by the move of the mobile station UE 1.
When the above assumed handover is caused, forwarding of user data, which accompanies the handover, is performed on a path p1 indicated by an arrow line of FIG. 2.
Namely, the user data forwarded from the gateway GW 5 to the relay node RN 2 via the first radio base station S-DeNB is returned from the relay node RN 2 to the first radio base station S-DeNB 3s. Then, the first radio base station S-DeNB 3s forwards the user data returned from the relay node RN 2 to the second radio base station T-DeNB 3t.
Upon completion of the handover, the second radio base station T-DeNB 3t transmits the user data forwarded from the first radio base station S-DeNB 3s to the mobile station UE 1 that has moved to the communication area of the second radio base station T-DeNB 3t.
FIG. 3 is an explanatory diagram of a second case where the handover control procedure standardized in 3GPP is applied.
The case illustrated in FIG. 2 is the case where the relay node RN 2 stays within the communication area of the first radio base station S-DeNB 3s and the mobile station UE 1 moves from the communication area of the relay node RN 2 to the communication area of the second radio base station 3t, causing an occurrence of a handover of the mobile station UE 1.
In the meantime, the case illustrated in FIG. 3 is the case where a handover occurs in the mobile station UE 1 due to a move of the mobile station UE 1 from the communication area of the first radio base station S-DeNB 3s to the communication area of the second radio base station T-DeNB 3t along with the relay node RN 2.
Assume that the handover control procedure, standardized in 3GPP, for the mobile station UE between the radio base stations, is applied in the case illustrated in FIG. 3 similarly to the case illustrated in FIG. 2. In this case, data forwarding that accompanies the handover is performed on a path p2 indicated by an arrow line of FIG. 3 in the handover case illustrated in FIG. 3.
Namely, the user data forwarded from the gateway GW 5 to the relay node RN 2 via the first radio base station S-DeNB is returned from the relay node RN 2 to the first radio base station S-DeNB 3s. Then, the first radio base station S-DeNB 3s forwards the user data returned from the relay node RN 2 to the second radio base station T-DeNB 3t. Moreover, the second radio base station T-DeNB 3t again forwards the user data forwarded from the first radio base station S-DeNB to the relay node RN 2 that has moved to the communication area of the second radio base station T-DeNB 3t.
As described above, forwarding of user data is performed to prevent a packet from being lost by a handover. Accordingly, forwarding of user data from and to the same relay node RN 2 as indicated by the path p2 of FIG. 3 is an unneeded process. Moreover, it cannot be said that forwarding of user data from and to the same relay node is an efficient use of a radio channel.